The present invention relates to a method for producing dual zone porous materials having an external zone bearing first silyl groups immobilized on the external surfaces of the porous material and internal zone having second silyl groups immobilized on the internal surfaces of the porous material and the dual zone materials so produced. More particularly, it relates to a method for catalyzed halosilylation of predominantly the external surfaces of a porous hydroxyl-bearing support and production of a dual zone material.
In copending application Ser. Nos. 154,754, and 598,120, there are disclosed dual surface (more properly termed dual zone) porous materials made by treating a porous hydroxyl-bearing support, such as porous silica, alumina, zirconia, etc., with substoichiometric amounts of an ultrafast silylating agent. This agent is chosen from those which are so reactive that the resulting surface group is immobilized in the external zone of the porous support before the agent has had time to migrate deeply into the porous internal zone. A subsequent silylation reaction can be employed to convert residual hydroxyl groups, which reside predominantly in the internal zone, to a second immobilized group of another type. See also, Williams & Tangney, Silanes, Surfaces & Interfaces, D. E. Leyden, ed., Gordon & Breach Publisher, 1986, P. 471 ff.
In copending application Ser. No. 598,120, the disclosed ultrafast silylating agents are reactive silane intermediates. In patent application Ser. No. 154,754 the ultrafast silylating agents are silanes having "leaving groups" such as (i) substituted amides, (ii) substituted amines, or (iii) thiothers. It is believed that these facile leaving groups lower the activation energy required for reaction with surface hydroxyl groups and thus enhance the extent to which the silane can be captured by covalent bond formation in the external zone of the porous material, that is, captured early during its diffusion path into said material.
As stated in copending application Ser. Nos. 154,754, and 598,120, traditional silylation reactions are generally not fast enough to permit preferential silylation of the external surface of the porous support. "Traditional silylation" is described in Plueddemann, Encyclopedia of Chemical Technology, 3rd ed., Vol. 20, page 962 et seq. Plueddemann states that silylation is the displacement of active hydrogen from an organic molecule by silyl groups where "The active hydrogen is usually OH, NH, or SH, and the silylating agent is usually a trimethylsilyl halide or a nitrogen-functional compound. A mixture of silylating agent is usually a trimethylsilyl halide or a nitrogen-functional compound. A mixture of silylating agents may be used; a mixture of trimethychlorosilane and hexamethyldisilazane is more reactive than either reagent alone, and the by-products combine to form neutral ammonium chloride."
Halosilanes, in particular, have been tried, but have been found to be insufficient to produce dual zone porous material. For example, M. L. Hunnicutt and J. M. Harris in "Reactivity of Organosilane Reagents on Microparticulate Silica." Anal. Chem., vol. 58, 1986, pp. 748-752 discuss the use of halosilanes in an attempt to demonstrate pore diffusion control of silica silylation. Hunnicutt and Harris discuss the results of competitive surface reactions between binary organosilane mixtures and silica gel. The organosilane mixtures used include mixtures of two haloalkylsilanes such as (1-bromomethyl)dimethylmonochlorosilane, (1-chloromethyl)dimethylmonochlorosilane, or (3-chloropropyl)dimethylmonochlorosilane, as well as mixtures of a haloalkysilane with an alkylsilane such as trimethylchlorosilane (TMCS) or hexamethyldisilazane (HMDS). In a number of instances a catalyst such as pyridine was added to the silica slurry prior to silane addition for base catalyzed reactions. Hunnicutt and Harris showed that their reaction did not display pore diffusion control. Thus they could not have produced dual zone materials (DZMs) with respect to differential distribution of their chosen immobilized groups. This outcome is believed to be due to several factors. Most importantly, mixtures of chlorosilanes of the type used by Hunnicutt and Harris do not react with sufficient speed and differentiality even when the reaction is catalyzed with pyridine.
Furthermore, the reaction conditions were not adjusted so as to produce DZMs even from the point of view of selective capture of both chlorosilanes together in the external zone. Firstly, the solvent they used was chloroform which is highly polar and is known to be a proton donor in hydrogen-bonded complexes. Such solvents have been found to reduce pore diffusion control, probably by sequestering the surface reactive sites (silanol) and thus slowing down the reaction rate. Protic solvents such as ethanol are even more deleterious since the halosilane is solvolyzed and transformed into the less reactive ethoxysilane. Secondly, Hunnicutt and Harris used excessive amounts of silane averaging 36 molecules/nM.sup.2 of silica surface, which would saturate the silica and thus afford a uniformly saturated surface treatment rather than a dual zone structure. Thirdly, the rate of silane addition to the silica slurry was excessively fast at about 0.3 molecules/nM.sup.2 /minute. Accordingly, individual silica particles would be subjected to unusual doses of silane and the resultant particle-to-particle heterogeneity would overcome any intraparticle inhomogeneity (dual zone structure) that might otherwise occur. Accordingly, been though Hunnicutt and Harris conducted what could be described as a catalyzed halosilane reaction, Hunnicutt and Harris do not teach one of ordinary skill in the art how to produce dual zone materials by means of such reaction mechanisms.
And yet, it is known to be desirable to produce dual zone porous materials having silyl groups of one type predominantly on the external surface and silyl groups of another type predominantly on the internal surface in order to provide on the external and internal surfaces differentially selective adsorbents, for example, for specific chromatographic and catalytic applications. It would also be desirable to do so with halosilane reactants which are relatively inexpensive and readily available. To date, however, it has not been possible to use halosilane reactants for this purpose.
Accordingly, the need remains for a method for halosilylation of predominantly the external surfaces of a porous hydroxyl-bearing support in the production of dual zone porous materials.